Many conventional photochromic materials, such as, for example, photochromic naphthopyrans, can undergo a transformation from a first form or state to a second form or state in response to the absorption of electromagnetic radiation. For example, many conventional thermally reversible photochromic materials are capable of transforming between a first “clear” or “bleached” ground-state form and a second “colored” activated-state form in response to the absorption of certain wavelengths of electromagnetic radiation (or “actinic radiation”). As used herein with reference to photochromic materials, articles and compositions, the terms “clear” and “bleached” mean the photochromic material, article, or composition is substantially without color, that is, has substantially no absorption of electromagnetic radiation within the visible region of the electromagnetic spectrum (420 nm-700 nm). As used herein the term “actinic radiation” refers to electromagnetic radiation that is capable of causing a photochromic material to transform from a first form or state to a second form or state. The photochromic material may then revert back to the clear ground-state form in response to thermal energy in the absence of actinic radiation. Photochromic articles and compositions that contain one or more photochromic materials, for example, photochromic lenses for eyewear applications, generally display optically clear and colored states that correspond to the photochromic material(s) that they contain. Thus, for example, eyewear lenses that contain photochromic materials can transform from a clear state to a colored state upon exposure to actinic radiation, such as certain wavelengths found in sunlight, and can revert back to the clear state in the absence of such radiation upon absorption of thermal energy.
When utilized in photochromic articles and compositions, conventional photochromic materials are typically incorporated into a host polymer matrix by one of imbibing, blending, and/or bonding. Alternatively, the photochromic material may be imbibed into a pre-formed article or coating. As used herein, the term “photochromic composition” refers to a photochromic material in combination with one or more other material, which may or may not be a different photochromic material.
For many photochromic applications, it is generally desirable to have a photochromic material that can rapidly revert from the colored, activated-state form to the clear, ground-state form, while still maintaining acceptable characteristics such as color density. For example, in photochromic eyewear applications, optical lenses comprising photochromic materials transform from an optically clear state to a colored state as the wearer moves from a region of low actinic radiation, such as indoors, to a region of high actinic radiation, such as into direct sunlight. As the lenses become colored, less electromagnetic radiation from the visible and/or ultraviolet regions of the electromagnetic spectrum is transmitted through the lens to the wearer's eyes. In other words, more electromagnetic radiation is absorbed by the lens in the colored state than in the optically clear state. When the wearer subsequently moves from the region of high actinic radiation back to a region of low actinic radiation, the photochromic material in the eyewear reverts from the colored, activated-state form to the clear, ground-state form in response to thermal energy. If this transformation from colored to clear takes several minutes or more, the wearer's vision may be less than optimal during this time due to the combined effect of the lower ambient light and the reduced transmission of visible light through the colored lenses.
Accordingly, for certain applications, it may be advantageous to develop photochromic materials that can more quickly transition from the colored form to the clear form, as compared to conventional photochromic materials. As used herein, the term “fade rate” is a measurement of the rate at which the photochromic material transforms from the activated colored state to the unactivated clear state. The fade rate for a photochromic material may be measured, for example, by activating a photochromic material to saturation under controlled conditions in a given matrix, measuring its activated steady state absorbance (i.e., saturated optical density) and then determining the length of time it takes for the absorbance of the photochromic material to decrease to one-half the activated steady state absorbance value. As measured in this fashion, the fade rate is designated by T1/2, with units of seconds.
Additionally, as mentioned above, typically the transformation between the ground-state form and the activated-state form requires that the photochromic material be exposed to certain wavelengths of actinic radiation. For many conventional photochromic materials, the wavelengths of actinic radiation that may cause this transformation typically range from 320 nanometers (“nm”) to 390 nm. Accordingly, conventional photochromic materials may not be optimal for use in applications that are shielded from a substantial amount of actinic radiation in the range of 320 nm to 390 nm. Therefore, for some applications, it may be advantageous to develop photochromic materials that can have a ground-state form absorption spectrum for electromagnetic radiation that is bathochromically shifted. As used herein, the term “bathochromically shifted” means having an absorption spectrum for electromagnetic radiation that is shifted to longer wavelength values. Thus a photochromic material that has a bathochromically shifted ground-state form absorption spectrum will require absorption of actinic radiation having a longer wavelength in order to transition from the ground-state form to the activated-state form.
For example, lenses for eyewear applications that are made using conventional photochromic materials may not reach their fully-colored activated-state form when used in an automobile. This is because a large portion of electromagnetic radiation in the range of 320 nm to 390 nm can be absorbed by the windshield of the automobile before it can be absorbed by the photochromic material(s) in the lenses. In certain applications, such as those involving behind the windshield use of photochromic materials, it may be advantageous if the ground-state form absorption spectrum of the photochromic material were bathochromically shifted such that the photochromic material may absorb sufficient electromagnetic radiation having a wavelength greater than 390 nm to permit the photochromic material to transform from the ground-state form to the activated-state form.
The absorption spectrum of a photochromic material in the activated-state form will correspond to the color of the medium or article containing the photochromic material, for example, the color of the eyewear lens, when exposed to actinic radiation. As specific wavelengths within the visible region of electromagnetic radiation are absorbed by a photochromic material in the activated-state form, the wavelengths within the visible region that are transmitted (i.e., not absorbed) correspond to the color of the photochromic material in the activated-state form. For example, absorption of wavelengths of light around about 500 nm to about 520 nm in the visible region of the electromagnetic spectrum results in a photochromic material that exhibits a “reddish” color, i.e., it absorbs visible radiation from the short wavelength or blue end of the visible spectrum and transmits radiation from the longer wavelength or red end of the visible spectrum. Conversely, absorption of wavelengths of light around about 580 nm to about 610 nm in the visible region of the electromagnetic spectrum results in a photochromic material that exhibits a “bluer” color, i.e., it absorbs visible radiation from the longer wavelength or red end of the visible spectrum and transmits radiation from the shorter wavelength or blue end of the visible spectrum.
Many current photochromic compounds have activated-state absorption spectrums that absorb visible light toward the blue end of the visible spectrum and exhibit a reddish color in the activated form. If the photochromic material has an activated-state absorption spectrum that is bathochromically shifted, i.e., shifted to absorb light having a longer wavelength, the photochromic material will exhibit a bluer color than the current photochromic material. For certain applications it may be desirable to have a photochromic material that has a bathochromically shifted activated form absorption spectrum for actinic radiation and which may therefore exhibit a bluer color.